Comment on “Masking and unmasking fishing down effects: The Bohai Sea (China) as a case study” (https://doi.org/10.1016/j.ocecoaman.2019.105033) by Cui Liang and Daniel Pauly

Abstract

The article “Masking and unmasking fishing down effects: The Bohai Sea (China) as a case study” (Liang and Pauly 2020) concluded that the mean trophic level of fishable biomass (MTL_b) declined more rapidly than the mean trophic level of catches (MTL_c) in the Bohai Sea since the 1950s. We have no doubt on the occurrence of a fishing down phenomenon or trophic cascades in the Bohai Sea, but argue that their comparative conclusion above is an artifact based on a flawed analysis of the relative biomass and catch data. First, they claimed a sharp decline in estimated MTL_b in the Bohai Sea based on bottom trawl survey data in only five years (i.e., 1959, 1982, 1992, 1998 and 2010) over half a century (1959–2010). We propose that changes in fishing vessels and gear types in those surveys after 1959 could make the estimated MTL_b in 1959 not comparable to those in the other years later. We also demonstrate that the declines in MTL_b from 1992 to 1998 and then to 2010 mainly resulted from the abrupt blooms of C. punctatus (a species with the lowest trophic level at 2.4 among all common or dominant fish and invertebrate species in the ecosystem) in 1998 and 2010. These two occasional declines based on three data points should be explained by large interannual variability in C. punctatus population dynamics, instead of trophic cascades or a fishing down effect which are ecological processes at system scale that usually only become apparent over a long time series like decades of years. Additionally, we illustrate that Liang and Pauly attributing the artifact of a much steeper decline in MTL_b than that in MTL_c to a masking effect for fishing down is unsupported by the data and literatures. Furthermore, we explain in details for our concerns on the accuracy of estimated marine fisheries catches data by taxa/species from China Fishery Statistical Yearbooks for any specific fishing area like the Bohai Sea, and suggest such data should be used with caution in future studies.